// 二叉树中的搜索
let root = [4,2,7,1,3], val = 2
var searchBST = function(root, val) {


    if (root == null) return null
    if(root.val == val) {
        return root
    }
   return searchBST(root.left, val) || searchBST(root.right, val) 
};